A carbon nanotube is a kind of fibrous carbon having a structure where graphene sheets are layered in an annual growth ring shape, and is a nanocarbon fiber having an extremely high aspect ratio where its diameter is 0.43 nm to several tens of nanometers, while its length reaches normally 100 nm to several hundreds of micrometers. The graphene sheet mentioned here indicates a layer where a carbon atom having an sp2 hybridized orbital, which forms a crystal of graphite, is located at the apex of each hexagon.
A carbon nanotube formed of one graphene sheet is called a single-walled carbon nanotube (hereinafter abbreviated as SWCNT). A carbon nanotube formed of two graphene sheets is called a double-walled carbon nanotube (hereinafter abbreviated as DWCNT). A carbon nanotube formed of three graphene sheets is called a triple-walled carbon nanotube (hereinafter abbreviated as 3WCNT). A carbon nanotube formed of four graphene sheets is called a quad-walled carbon nanotube (hereinafter abbreviated as 4WCNT). A carbon nanotube formed of five graphene sheets is called quint-walled carbon nanotube (hereinafter abbreviated as 5WCNT). A carbon nanotube formed of six or more graphene sheets is often called, in sum, multi-walled carbon nanotube (hereinafter abbreviated as MWCNT). Hereinafter, for indicating the plural number of the carbon nanotube, it is expressed by adding “s” meaning the plural number as in SWCNTs.
On a cylindrical surface of the graphene sheet, a formation is shown in which a hexagonal mesh that is formed by carbon atoms is rolled up in a spiral form and the spiral state is called chirality. It is known that various kinds of physical properties of the carbon nanotube differ in response to the number of graphene sheet layers, diameter of tube, and chirality.
A carbon nanotube manufacturing method uses a CVD method using organic compound vapor as a raw material, an arc discharge method using a graphite electrode, a laser evaporation method, a liquid phase synthesis method, etc.
For example, in Nikolaev P and other six writers, “Gas-phase catalytic growth of single-walled carbon nanotubes from carbon monoxide,” Chemical Physics Letters, vol. 313, pp. 91-97, 1999 (Document 1), there is described a method for manufacturing SWCNTs by disproportionation of carbon monoxide under a high pressure using iron particles as a catalyst, and only SWCNTs can be selectively manufactured according to this method.
Furthermore, in S. Maruyama and other six writers, “Growth of vertically aligned single-walled carbon nanotube films on quartz substrates and their optical anisotropy,” Chemical Physics Letters, vol. 385, pp. 298-303, 2004 (Document 2), there is disclosed method for selectively forming only SWCNTs on a substrate by a CVD method using ethanol as a raw material.
On the other hand, as a method for manufacturing DWCNTs, in Hisanori Shinohara and other six writers, “Synthesis of high purity double wall carbon nanotubes by the CCVD method using zeolites,” Conference Abstract Book of 26th Fullerene Nanotubes General Symposium, Fullerene and Nanotubes Research Society, p. 100, Jan. 7, 2004 (Document 3), there is disclosed a method for manufacturing a carbon nanotube mixture containing 50% to 60% DWCNTs by a CVD method using metallic fine particles as a catalyst carried in zeolites and using an alcohol as a raw material.
Additionally, as means for checking the number of graphene sheet layers of the carbon nanotubes which those skilled in the art normally use, there is generally used a method in which individual carbon nanotubes are observed by a transmission electron microscope to count, on observation images, the number of images of graphene sheet layers which appear on a wall surface of the carbon nanotube. Means for estimating a proportion of a carbon nanotube having a predetermined number of layers to an assembly of numerous carbon nanotubes is as follows. Specifically, the assembly is observed by the transmission electron microscope. Then, the number of layers of each of the numerous carbon nanotubes extracted at random is determined. The proportion is estimated as the number of carbon nanotubes having a predetermined number of layers with respect to the total number of carbon nanotubes thus extracted.